Signal synchronization is important in wireless telecommunication. In modem systems, there are various levels of synchronization, such as, carrier, frequency, code, symbol, frame and network synchronization. At each level, synchronization can be divided into two phases: acquisition (initial synchronization) and tracking (fine synchronization).
A typical wireless communication system, such as specified in the 3rd Generation Partnership Project (3GPP), sends downlink communications from a base station to one or a plurality of User Equipments (UEs) and uplink communications from UEs to the base station. A receiver within each UE operates by correlating, or despreading, a received downlink signal with a known code sequence. The code sequence is synchronized to the received sequence in order to get the maximal output from the correlator.
A receiver may receive time offset copies of a transmitted communication signal known as multi-path. In multi-path fading channels, the signal energy is dispersed over a certain amount of time due to distinct echo paths and scattering. To improve performance, the receiver can estimate the channel by combining the multi-path copies of the signal. If the receiver has information about the channel profile, one way of gathering signal energy is then to assign several correlator branches to different echo paths and combine their outputs constructively. This is conventionally done using a structure known as a RAKE receiver.
Conventionally, a RAKE receiver has several “fingers”, one for each echo path. In each finger, a path delay with respect to some reference delay, such as the direct or the earliest received path, must be estimated and tracked throughout the transmission. The estimation of the path's initial position in time may be obtained by using a multi-path search algorithm. The multi-path search algorithm does an extensive search through correlators to locate paths with a desired chip accuracy. RAKE receivers are able to exploit multi-path propagation to benefit from path diversity of transmitted signals. Using more than one path, or ray, increases the signal power available to the receiver. Additionally, it provides protection against fading since several paths are unlikely to be subject to a deep fade simultaneously. With suitable combining, this can improve the received signal-to-noise ratio, reduce fading and ease power control problems.
In the context of mobile UEs, due to their mobile movement and changes in the scattering environment, the delays and attenuation factors used in the search algorithm change as well. Therefore, it is desirable to measure the tapped delay line profile and to reallocate RAKE fingers whenever the delays have changed by a significant amount.
An important design problem of a RAKE receiver is how to accurately search and find multiple signal paths. There are several key parameters to be optimized for the receiver system, such as mean acquisition time, optimum threshold setting, probabilities of detection and false alarm, etc. One problem with a RAKE receiver is that the paths can disappear or may not be detected by a RAKE location process. Therefore, there exists a need for an improved receiver.
Another severe design problem of a RAKE receiver is that it is not always possible to separate the received energy into components due to distinct multipath components. This may happen, for example, if the relative delays of the various arriving paths are very small compared to the duration of a chip. Such situations often arise in indoor and urban communication channels. The problem is often referred to as the “Fat Finger Effect.”
While there exist techniques for demodulating the data from Fat fingers, in order to apply such techniques the received energy belonging to a Fat finger must be identified. Unfortunately, typical RAKE correlators are designed to search for distinct single-path components in a multipath channel and are unable to perform this identification. Thus, there exists a need for a receiver capable of identifying the Fat fingers.